A polymer electrolyte fuel cell (PEFC) is highly possible to be applied as a stationary domestic power generation system or in an electric car, and a PEFC system requiring a fuel consisting of a hydrogen-rich gas (concentration of H2>35%) with a CO concentration lower than 20 ppm. A hydrogen-rich reformate gas formed from a reforming reaction of hydrocarbon contains about 4˜15% of CO, which needs to undergo a water-gas shift (WGS) reaction to reduce the CO content to less than 1% of, followed by a preferential oxidation reaction or a methanation reaction and a preferential oxidation reaction in serial in order to reduce the CO concentration to be less than 100 ppm, or even less than 20 ppm. By selecting a suitable catalyst and controlling the reaction at a suitable temperature, the CO contained in a hydrogen-rich reformate gas can be converted to methane through the methanation reaction, and thus reduce the CO concentration therein. One advantage of the methanation reaction is the reactor design is simpler than that for use in the PrOX reaction. However, one defect of the methanation reaction is removing one mole of CO requiring depleting three moles of hydrogen. Thus, methanation mainly is applied on a reformate gas with a low CO concentration or for miniaturization of a fuel reformer. At present, methanation has been used in the design of reformers by the Osaka Gas Company and the Mercedes-Benz Automobile Company.
However, other than catalyzing a CO methanation reaction, a methanation catalyst will also catalyze a CO2 methanation reaction. In order to taking into account of both CO removal and hydrogen loss, a good methanation catalyst should have good catalytic activity and reaction selectivity to the CO methanation reaction.CO+3H2→CH4+H2OCO2+4H2→CH4+2H2O (side reaction)
An active metal used in the methanation reaction catalyst for the conventional petrochemical industry mostly is nickel. A nickel-catalyzed methanation reaction has a slightly higher reaction temperature of about 400° C. When the reactant composition contains CO2, a nickel catalyst at 400° C. is liable to catalyze a CO2 methanation reaction, which will consume a larger amount of hydrogen and can not be used in serial to an existing WGS reaction. Other than a nickel catalyst, ruthenium is most commonly used as an active metal in a methanation catalyst.
U.S. Pat. No. 3,787,468 discloses a mixed Ru—WOX and Pt—Ru—WOX catalyst, which are applicable on methanation of CO and CO2, wherein Ru—WOX has a better methanation activity, and Pt—Ru—WOX has a lower activity. Said catalysts contain Ru as a main ingredient, Pt in an amount of 0-50% of the amount of Ru, and WOX in an amount of 5-20% of Ru. That is said catalysts contain a high content of precious metal, which leads to a high production cost.
U.S. Pat. No. 3,615,164 discloses a Ru or Rh catalyst suitable for selective methanation of CO, wherein said Ru or Rh is supported on a metal oxide carrier.
In comparison with a nickel catalyst, a ruthenium catalyst has a lower reaction temperature in catalyzing a methanation reaction. However, the reaction temperature thereof is deeply influenced by a space velocity. Even though a ruthenium catalyst has the advantages of a high activity in catalyzing a CO methanation reaction and a low reaction temperature (U.S. Pat. No. 3,615,164; U.S. Pat. No. 3,787,468), ruthenium is liable to form a Ru(CO)x complex with CO, whereas the Ru(CO)x complex will sublimate in the methanation reaction, causing deterioration of the catalyst activity, thereby affecting the lifespan of the catalyst.